This invention relates in general to wellbore logging devices and in particular to wellbore logging devices suitable for the detection of low energy photons. Still more particularly, this invention relates to logging devices suitable for the detection of low energy photons which include an insulating flask for providing thermal insulation for the enclosed photon detection circuit and which permit transmission of low energy photons.
Radioactivity well logging devices are well known in the prior art. Such devices are typically utilized to determine the bulk density and photoelectric cross-sectional index of an earth formation which is transversed by a wellbore. Logging devices for determining bulk density are typically based upon the principle of gamma-ray scattering. Present devices typically employ two independently spaced gamma-ray photon detectors in conjunction with a photon emitting source such as a Cesium 137 isotope. The detectors and source are housed in a pressure type vessel and are lowered into a wellbore where they are forced into contact with the wall of the wellbore. The sondes employed for this measurement are subjected to all of the environmental conditions present during the logging opeation which include elevated temperatures and pressures.
It is not uncommon for such logging devices to encounter temperatures approaching 400.degree. Fahrenheit. At these elevated temperatures the accuracy of the tool measurement becomes increasingly influenced by degradation associated with such temperatures on the outputs of the photon detectors and the associated electronic circuitry. Additionally, continuous operation of the gamma-ray detection assembly at elevated temperatures may result in a shortening of the life span of such assemblies and may ultimately result in failure of the circuitry.
One method of protecting the temperature sensitive components of such a logging device is to house these components within an insulating storage vessel such as a Dewar flask. This is a well known practice and is typically utilized in the wellbore environment to retard heat flow from the environment into the heat sensitive components, thus permitting extended operation of the tool relative to a similar tool without an insulating storage vessel.
Construction of known Dewar flasks providing insulation for such logging tools typically utilizes materials having low thermal conductance such as stainless steel or titanium. However, it is equally important not to disrupt the broad range of scattered photon radiation which is incident to the loggin device's detectors. The scattered photon energy is utilized to describe the parameters of the earth formation surrounding the wellbore and must be measured with maximum possible accuracy in order to accurately determine those parameters. The utilization of known low thermal conductance materials such as titanium and stainless steel may shield or block low energy photons from reaching an insulated photon detector and may thus render the measurement of scattered photon radiation inaccurate. Thus, it should be evident that a need exists for a logging device which can operate successfully in a high temperature environment and which does not significantly disrupt the transmission of scattered gamma-ray photons.